In prior arts, there has been used an ellipsometer or the like to measure optical characteristics of a substrate for an optical disk. In such a device, a known polarized bundle of parallel rays irradiates on a disk to be measured and the passed or reflected rays pass through an analyzer in a rotary condition whereby the optical characteristics of the disk are determined by measuring the polarization of the rays.
In practical cases of the disk used as a storage medium, the incident light rays of the disk are not in the form of parallel rays, but in the form of collected rays. Thus, it will be noted that it is very difficult to measure an effect by the optical characteristics of the disk taken on practical playback signals from the measurement result by the ellipsometer.
Since the diameter of the bundle of parallel rays of the ellipsometer cannot be as small as the width of the practical track formed on the disk, it is difficult to measure the effect of variation on the optical characteristics over the fine region which is taken on the practical playback signals. Furthermore, the ellipsometer disadvantageously has a large configuration in case that it is tried to improve the accuracy of the optical axis thereof and also has an adjustment made difficult when the disk to be measured is mounted with a longer time also taken for the adjustment.